Phosphors comprising silicates, phosphates (for example, apatite) and aluminates as host materials, with transition metals or rare earth metals added as activating materials to the host materials, are widely known. As blue LEDs, in particular, have become practical in recent years, the development of white light sources utilizing such blue LEDs is being energetically pursued. As white LEDs are expected to have lower power consumption and longer usable lives than existing white light sources, development is progressing toward their applications in backlights of liquid crystal panels, indoor lighting fixtures, backlights of automobile panels and the like.
The currently developed white LEDs generally comprise YAG (yttrium-aluminum-garnet)-based phosphors coated on the surfaces of blue LEDs, with Ce-activated YAG-based phosphors converting the blue light of the blue LEDs to yellow light. A portion of the blue light of 450 nm wavelength emitted by the blue LED penetrates the phosphor layer, while the remainder strikes the phosphor and is converted to yellow light. The blue and yellow colors combine to appear as white light.
YAG-based phosphors, however, are associated with the problem of emission of blue-tinted white light due to a lower intensity when the excitation wavelength exceeds 400 nm, as well as a low light-emitting efficiency because the excitation energy of the blue LED does not match the excitation energy of the YAG-based phosphor. Another problem is inadequate durability of the coated phosphor layer. An improvement has therefore been desired in the performance of phosphors used for wavelength conversion.
In recent years, oxynitride phosphors such as oxynitride glass, β-sialon, α-sialon and the like having nitrogen in the structure have received attention as structurally stable substances capable of shifting excitation light or emitted light toward the long wavelength end. Sialon with α- or β-Si8N4 crystal structure is a solid solution composed of mainly Si, Al, N and O atoms. Two forms thereof are known, β-sialon and α-sialon, which are both materials with excellent abrasion resistance and corrosion resistance.
The α-sialon is a solid solution with double substitution of Al for Si position and O for N position of α-silicon nitride, with a modifying cation M occupying at the interstitial sites. Chemical composition of α-sialon is represented by the general formula: MxSi12−(m+n)Al(m+n)OnN16−n (wherein 0.3≦x<1.5, 0.3≦m<4.5, 0<n<2.25, and m=ax where a is the valence of the metal M). Because α-sialon has high hardness and excellent abrasion resistance, various processes for producing sialon-based sintered bodies have been investigated. Li, Mg, Ca, Y and lanthanide metals are known as metals M which dissolve in α-silicon nitride lattice and stabilize its structure, occupying the interstitial sites of the unit lattice of α-silicon nitride.
La and Ce, which are lanthanide metals with large ionic radii, are considered to poorly occupy the interstitial sites of α-silicon nitride. However, simultaneous addition of La or Ce with the aforementioned metal elements has been carried out in an attempt to produce solid solutions with La or Ce occupying the interstitial sites of α-silicon nitride. For example, the Journal of European Ceramic Society, Vol.8, pp.3–9(1991) reports that simultaneous addition of Ce with Y yielded α-sialon having both Ce and Y stabilized therein. Also, the Journal of Materials Science Letters, Vol.15, pp.1435–1438 reports that by rapid cooling from high temperature it is possible to synthesize α-sialon stabilized with Ce (Ce-α-sialon). However, the Ce-α-sialon content is only about 20% while the remaining major portion is β-sialon, and partial formation of a 21R phase, a polytype of AlN, has been confirmed. Moreover, Japanese Unexamined Patent Publication No. 2001-261447 discloses a sialon-based sintered body with high hardness and excellent abrasion resistance, comprising only β-sialon and a grain boundary glass phase in addition to Ce-stabilized α-sialon.
Nevertheless, the aforementioned disclosed technologies all relate to production of sialon-based sintered bodies, and are not directed toward powder production nor toward production of oxynitride phosphors. Furthermore, no reference is made in any way to the effects of the metal impurity concentrations of the obtained sialon-based sintered bodies.
On the other hand, Japanese Unexamined Patent Publication No. 2002-363554 discloses an oxynitride phosphor with an activated rare earth element, represented by the general formula: MxSi12−(m+n)Al(m+n)OnN16−n: Re1yRe2z (wherein 0.3<x+y+z<1.5, 0.01<y<0.7, 0≦z<0.1, 0.3<m<4.5, 0≦n<1.5), and having all or a portion of the metal M dissolved in α-sialon (where M is at least one type of metal selected from among Ca, Mg, Y or lanthanide metals excluding La and Ce) replaced with two different lanthanide metals, specifically a lanthanide metal Re1 as the luminescence center (where Re1 is at least one type of lanthanide metal selected from among Ce, Pr, Eu, Er, Tb and Yb) or a lanthanide metal Re2 as the co-activator with the lanthanide metal Re1 (where Re2 is Dy). However, this publication not only lacks any mention regarding the powder properties such as purity, crystal phase and particle size distribution of the rare earth element-activated oxynitride phosphor, but it is also completely unconcerned with properties such as purity, crystal phase and particle size distribution of the Si3N4, AlN, alkali metals and rare earth metals used as the starting materials for preparation of the oxynitride phosphor.
The present applicant has proposed, in Japanese Unexamined Patent Publication (Kokai) SHOWA No. 62-223009, a process for production of α-sialon powder using as the starting materials (a) amorphous silicon nitride powder, (b) metallic aluminum or aluminum nitride, (c) an oxide of a metal which interstitially dissolves in the α-sialon lattice as a solid solution or a metal salt which produces the metal oxide upon thermal decomposition, and (d) an aluminum or silicon compound containing oxygen.
However, while some effect is achieved to develop α-sialon-based oxynitride with excellent mechanical properties as an abrasion-resistant material, the properties are inadequate from the standpoint of its use as an optical material and, therefore, further improvement has been necessary before it can be practicably utilized.
It is an object of the present invention to provide an oxynitride phosphor composed mainly of rare earth element-activated α-sialon represented by the general formula: MxSi12−(m+n)Al(m+n)OnN16−n:Lny (wherein 0.3≦x+y<1.5, 0<y<0.7, 0.3≦m<4.5, 0<n<2.25, and m=ax+by, where a is the valence of the metal M and b is the valence of the lanthanide metal Ln), as a photoluminescent phosphor capable of realizing the high brightness of a white LED using a blue LED as the light source, as well as a process for its production.